In the search for high performance materials, considerable interest has been focused upon carbon fibers. The term "carbon fibers" is used herein in its generic sense and includes graphite fibers as well as amorphous carbon fibers. Graphite fibers are defined herein as fibers which consist essentially of carbon and have a predominant x-ray diffraction pattern characteristic of graphite. Amorphous carbon fibers, on the other hand, are defined as fibers in which the bulk of the fiber weight can be attributed to carbon and which exhibit an essentially amorphous x-ray diffraction pattern. Graphite fibers generally have a higher Young's modulus than do amorphous carbon fibers and in addition are more highly electrically and thermally conductive.
Industrial high performance materials of the future are projected to make substantial utilization of fiber reinforced composites, and carbon fibers theoretically have among the best properties of any fiber for use as high strength reinforcement. Among these desirable properties are corrosion and high temperature resistance, low density, high tensile strength, and high modulus. Graphite is one of the very few known materials whose tensile strength increases with temperature. Uses for carbon fiber reinforced composites include aerospace structural components, rocket motor casings deep-submergence vessels, and ablative materials for heat shields on re-entry vehicles.
In the prior art numerous materials have been proposed for use as possible matrices in which carbon fibers may be incorporated to provide reinforcement and produce a composite article. The matrix material which is utilized is commonly a thermosetting resinous material or metal.
While it has been possible in the past to provide carbon fibers of highly desirable strength and modulus characteristics, difficulties have arisen when one attempts to gain the full advantage of such properties in the resulting carbon fiber reinforced composite article. Such inability to capitalize upon the superior single filament properties of the reinforcing fiber has been traced to inadequate bonding between the fiber and the matrix in the resulting composite article.
Heretofore, composite articles produced by the incorporation of graphite fibers in a magnesium matrix have been projected to hold the potential of offering the highest specific strength of any metallic structural material. See, for instance "Metallic Matrix Composites" edited by Kenneth G. Kreider, Vol. 4, Page 381 (Academic Press. 1974). However, it has been found that substantial bonding difficulties between the carbon fiber reinforcement and the metallic matrix have been encountered when one has attempted to utilize a metal matrix material which is a magnesium containing metal. Carbon fibers normally are not wetted to any significant degree by molten magnesium or magnesium alloys. Poor adhesion between the fiber reinforcement and the matrix is the consequence.
Various techniques have been proposed in the past for modifying the fiber properties of a previously formed carbon fiber via an intermediate process in order to make possible improved adhesion when present in a composite article. For instance in the prior art techniques it has been proposed to overcome carbon fiber bonding difficulties by precoating the carbon fibers prior to introducing them into molten magnesium. According to such proposals the wettability of such fibers by molten magnesium previously has been enhanced by precoating with titanium via plasam spraying or physical or chemical vapor deposition, or by electroplated or electroless nickel. Such precoating techniques have proven to be highly time consuming and expensive. The difficulties encountered when coating carbon yarn or tow on a large scale, particularly when done continuously, are enormous. The available coating materials which will not react chemically with the carbon fiber are limited, e.g. titanium and boron. The equipment utilized for vapor precoating must be air and vacuum tight which is difficult to reliably accomplish in a continuous operation. The precoating thickness is difficult to control and generally is rather high, e.g. one micrometer or more in thickness. As a result if the coating forms a substantial portion of the metal matrix, this commonly is detrimental to composite properties. The formation of carbon fiber reinforced composites employing a magnesum containing metal matrix accordingly has been limited in the prior art in spite of the outstanding property potential offered by the resulting composite article if good adhesion between the fiber reinforcement and matrix can be achieved.
It is an object of the present invention to provide an improved process for the production of carbon fiber reinforced magnesium composite articles.
It is an object of the present invention to provide an improved process for the production of carbon fiber reinforced composite articles which renders the carbon fiber reinforcement readily wettable by a molten magnesium containing metal.
It is an object of the present invention to provide a process for the production of carbon fiber reinforced composite articles which overcomes bonding difficulties between carbon fiber reinforcement and a magnesium containing metal matrix encountered in the prior art.
It is an object of the present invention to provide an improved process for the production of carbon fiber reinforced magnesium composite articles which eliminates the need for an intermediate carbon fiber precoating step.
It is an object of the present invention to provide an improved process for the production of carbon fiber reinforced magnesium composite articles exhibiting highly satisfactory mechanical properties especially in the area of enhanced fiber strength translation in the composite which may be carried out on an expeditous and inexpensive basis.
It is an object of the present invention to provide a process for the production of a carbon fiber reinforced magnesium composite which exhibits an improved compressive and shear strength.
It is another object of the present invention to provide improved composite articles wherein carbon fiber reinforcement is incorporated in a magnesium containing matrix using a fiber-wetting magnesium compound as a promoter of adhesion between the carbon fiber reinforcement and the magnesium-base matrix.
These and other objects as well as the scope, nature, and utilization of my invention will be apparent to those skilled in the art from the following description and claims.